1. Field of the Invention
The present invention relates to a shoe press belt for making paper, and in particular, to a closed-type shoe press belt and a method for manufacturing the same.
2. Discussion of the Prior Art
The press part of a paper-making machine may be an open-type shoe press or a closed-type shoe press. Since the former open-type shoe press needs a larger installation space and has a drawback of oil scattering, the latter closed-type shoe press has been mainly used in recent years.
A closed-type shoe press is shown in FIG. 8 and FIG. 9. In the drawings, reference numeral 21 designates a press roll, reference numeral 22 designates a shoe, and reference numeral 23 designates a shoe press belt. The shoe press belt 23 is formed in an endless shape and runs with an upper felt 25 and a lower felt 25' which pinch a wet paper sheet 24, and the wet paper sheet 24 pinched between the felts is pressed by the press roll 21 and the shoe 22 to squeeze water from the wet paper sheet 24.
The shoe press belt 23 described above is moved by a press roll 21 as a drive source via the felt 25, the wet paper sheet 24 and the felt 25'. When the shoe press belt 23 is moved, it is bent to the roll side at both edges of the shoe press belt 23 extending off from both edges of the press roll 21, but it is deformed in a curve in an opposite direction in the part between edges 22a and 22b of the shoe 22 in the direction of MD, that is, in the part where it is pressed by the shoe 22 and the press roll 21. Therefore, the complicated bending applies a compressive force and a tensile force to both ends of the shoe press belt 23 inside of and outside of the boundary at both ends of the shoe 22 in the direction of CMD.
That is, in the case where the shoe press belt 23 is moved in the direction of the arrows shown in FIG. 9, there is a pressed part 23a (shown by hatching) and an unpressed part 23b (shown by blank) and a drive force is applied to the pressed part 23a and the unpressed part 23b tends to lag. Therefore, this produces stress S at the boundary parts 23c of the belt 23, which causes distortions and cracks.
A closed-type shoe press belt 23 having a short circumferential length (size) and hence severe usage conditions, as described above, needs to be durable, and hence is reinforced with a base fabric inserted into the thick portion of the belt. Typical examples of the closed-type shoe press belt are disclosed in Japanese Published Examined Patent Application No. 3-57236, Japanese Published Unexamined Patent Application No. 64-45888, Japanese Published Unexamined Patent Application No. 64-45889, Japanese Published Unexamined Patent Application No. 1-503315, Japanese Published Unexamined Patent Application No. 1-29892, Translation of Unexamined PCT Application No. 5-505428.
The Japanese Published Examined Patent Application No. 3-57236 discloses a shoe press belt made by mounting an endless base fabric around a mandrel and then by flowing resin thereon, that is, by a die molding method. This method has a merit of producing a uniform thickness and a smooth surface but has a problem in setting a position of the base fabric uniformity in the direction of thickness.
Further, the Japanese Published Unexamined Patent Application No. 64-45888 discloses a shoe press belt made by forming a first resin layer around a mandrel, then by putting a thermally contractible base fabric on the first resin layer, further by applying resin on the base fabric and by drying and curing the resin to form a second resin layer. This method can set a position of the base fabric uniformily in the direction of thickness, but since the base fabric used in this case is a fabric having an end, it is difficult to position the base fabric in the direction of MD and hence a pin hole might be produced.
Further, the Japanese Published Unexamined Patent Application No. 64-45889 discloses a shoe press belt made by mounting a not-yet-cured resin sheet around a mandrel, by putting a thermally contractible ground fabric on the resin sheet, further by winding a second not-yet-cured resin sheet on the base fabric, and finally by winding a ribbon made of thermally contractible resin on the second resin sheet and by thermally contracting the ribbon. Also in the case, as is the case with the Japanese Published Unexamined Patent Application No. 64-45888, since the base fabric used in this case is a fabric having an end, it is difficult to position the base fabric in the direction of MD and hence a pin hole might be produced.
Still further, the Japanese Translation of Unexamined PCT Application No. 1-503315 (U.S. Pat. No. 5,134,010) discloses a shoe press belt made by looping yarns in the axial direction of a mandrel around the whole periphery of the mandrel at constant intervals without using a fabric as a base fabric, by winding a yarn spirally thereon in the direction of MD, and then by flowing resin thereon. This method has a merit that a change in size caused by tension in the direction of CMD is made small, but has a problem that the yarns in a longitudinal direction and a yarn in the lateral direction constitute different layers and do not crimp the other yarn each other to reduce the flexibility of the base fabric and to change the center of bending in the direction of MD and in the direction of CMD, whereby bending stress applied by the bending to the obverse and reverse resin layers becomes large and trends to produce cracks at a part of the belt contacting the ends of the shoe.
Still further, the Japanese Published Unexamined Patent Application No. 1-298292, discloses a shoe press belt made by impregnating a mat-shaped fiber belt with not-yet-cured resin, by winding the fiber belt on a mandrel spirally, and by heating and curing the fiber belt. This belt has a problem that it is not stable in size in the direction of MD and in the direction of CMD and that it is apt to be separated.
Still further, the Japanese Translation of Unexamined PCT Application No. 5-505428 (U.S. Pat. No. 5,062,924) discloses a shoe press belt made by impregnating a fabric made of yarns, which have a low elastic modulus and are arranged in the direction of MD, and a yarn, which has a high elastic modulus and is wound in the direction of CMD, with not-yet-cured resin, by winding the fabric on a mandrel spirally, and by curing the fabric. This has a problem that the belt might be separated when stress is applied to a part where the fabric wound on the mandrel overlaps.
As described above, a shoe press belt and a reinforcing structure thereof produced by the conventional methods disclosed in the Japanese Published Examined Patent Application No. 3-57236 and others described above had a difficult problem that they could not show sufficient base material characteristics placing emphasis on durability required of the belt itself, and bending strength and tension in the direction of MD, which are required for a belt driven by a felt in range of the width of the belt in the direction of MD of a shoe.